In the field of mobile or wireless communication, different types of radio networks have been developed to provide radio access for various wireless devices. The radio networks are constantly developed to meet demands from subscribers using increasingly advanced services and devices such as smartphones and tablets, which may require considerable amounts of bandwidth and resources for data transport in the networks. It is therefore of interest to improve and optimize the capacity and performance in such radio networks, e.g. by utilizing available radio resources efficiently for radio communication. Furthermore, it is desirable to reduce energy consumption in both the network and the wireless devices, as well as the amount of interference generated by transmissions made by wireless devices and access points in the network, which in turn could enhance both capacity and performance.
In this disclosure, the term “wireless device” is used to represent any communication entity capable of radio communication with a radio network by sending and receiving radio signals, such as e.g. mobile telephones, tablets, laptop computers and Machine-to-Machine, M2M, devices. Another common generic term in this field is “Station, STA” which is also sometimes used herein instead of wireless device, and the terms STA and wireless device are thus interchangeable throughout this disclosure. Further, the term “access point”, is used herein to represent any node of a radio network that is operative to communicate radio signals with wireless devices. The access point discussed herein could also be referred to as a network node, base station, radio node, e-NodeB, eNB, NB, base transceiver station, etc., depending on the type of network and terminology used.
For example, multiple wireless devices may be served by an access point for receiving data which may have been accumulated in a buffer at the access point, which is referred to as “pending data” in this description.
A typical scenario is that downlink (DL) data reception requires the wireless device to first receive an indication from the network that there is pending DL data to be received, and then perform an uplink (UL) transmission such as a request for transmission or the like, prior to be able to receive that data. In order to achieve low power consumption in the wireless device, it has been suggested in the Third Generation Partnership project, 3GPP, that contention-based uplink transmission can be employed instead of requiring access reservation for each communication, thus omitting the energy-consuming access reservation process. Some examples of how contention-based uplink transmissions can be employed are described in WO 2010057540 A1.
Contention-based uplink transmission generally means that any wireless device can transmit data or a message to an access point on a shared radio channel without reserving radio resources in advance, at the risk of collision when two or more wireless devices happen to transmit simultaneously such that the access point is not able to decode the colliding transmissions. When not transmitting, the wireless devices can enter sleep mode to save power and wake up again when it is time for next transmission. This type of contention-based scheme is employed e.g. in Wireless Local Area Network, WLAN, systems according to the standard document IEEE 802.11ah, Draft 2.0 and the mechanism of listening to the radio channel before transmitting is referred to as Carrier Sense Multiple Access, CSMA/Collision Avoidance, CA.
The embodiments and examples described herein relate to the IEEE802.11ah standard (D3.0 version) in particular but may be generalized to other wireless systems as well where downlink (DL) data reception requires the device or station (STA) to first receive an indication from the network that there is pending data to be received on the DL and then perform an uplink (UL) transmission, e.g. by sending a PS-Poll or the like, prior to be able to receive that data.
IEEE802.11ah is a new amendment to the 802.11 (2012) standard specifying wireless local area networks (WLAN), also known as Wifi. The WLAN system uses Industrial, Scientific and Medical (ISM) bands, to access the wireless medium via contention based protocols complying with different regulatory domains.
An element in the 802.11ah medium access control (MAC) protocol is the Traffic indication map (TIM) enabling the use of power save (PS) mode. Wireless devices, also referred to as TIM stations, listen to the access point (AP) transmitting beacons with indications of pending DL data, sometimes referred to as “TIM beacons” in this disclosure. At every beacon interval, the AP shall assemble a partial virtual bitmap containing the buffer status per destination for STAs in the power save mode, and the AP shall indicate the buffer status, i.e. pending data, for the STAs in the TIM field of the Beacon frame [see e.g. 10.2.1.6, 802.11]. Once an indication of pending data is received, the STA needs to reply to the AP by transmitting a PS-Poll, to indicate that the STA is available to receive the data in the DL.
STAs may operate in active mode, power save mode and automatic power save delivery mode (APSD) [see e.g. 10.2.1.9, 10.2.1.10, 10.2.1.11 in 802.11].
When operating in active mode, the STAs have their receiver activated continuously, thereby such STAs do not need to interpret the TIM elements in the beacon frame. Also, the AP will not buffer data addressed for STA operating in active mode [see e.g. 10.2.1.6 and 10.2.1.11 in 802.11].
When operating in power save mode, a STA detects that the bit corresponding to its association identifier (AID) is 1 in the TIM, and the STA shall therefore issue a PS-Poll.
For STAs operating in Automatic Power Save Delivery mode (APSD), two cases may occur as follows:                If a scheduled single service point (SP) has been setup, the STA wakes up at its scheduled start time and the AP sends the DL data.        If the STA is initiating an unscheduled SP, the STA wakes up and transmits a so-called trigger frame to the AP. In response thereto, the AP transmits DL data to the corresponding STA. When one or more Access Categories, ACs are not delivery-enabled, the STA may retrieve Bufferable Units, BUs using those ACs by sending PS-Poll frames to the AP.        
The STAs are typically divided into groups such that the beacons with indications of pending DL data are transmitted for one group at a time. The TIM is communicated by means of a Delivery TIM (DTIM) beacon and a TIM beacon per TIM group. The DTIM beacon is sent out periodically by the AP according to a chosen period which could be in the range of 1 ms and ˜60 seconds. The TIM beacons are distributed evenly throughout the DTIM period with a predefined formula [see e.g. 10.2.2.3 and 10.2.2.4 from 802.11ah draft]. There may be up to 8 TIM groups which are addressed by corresponding TIM beacons. The TIM beacon for the first TIM group may be sent together with the DTIM beacon.
An example of how DTIM and TIM beacons may be configured and transmitted by an access point over time in the above manner is illustrated in FIG. 1 involving four groups 1-4 of wireless devices or STAs. A first STA group is addressed by a TIM beacon 101a transmitted together with a DTIM beacon, followed by a TIM period 101b when the STAs in the first group can transmit a PS-poll and then receive data from the access point, before the next STA group is addressed by a next TIM beacon 102a. Thus, each TIM group is addressed by a TIM beacon 101a, 102a, 103a, 104a which are followed respective periods 101b, 102b, 103b, 104b that can be used for PS-polls and data reception by the respective STA groups. When all groups 1-4 have been addressed with TIM beacons and TIM periods, the illustrated sequence or cycle is repeated, as shown in the figure. Thus, a wireless device gets one opportunity per sequence or cycle to receive pending data provided that the wireless device manages to get a PS-Poll across to the access point after an indication of pending data in a TIM beacon.
The limited time reserved for a single TIM group clearly limits the possibilities to reach the STA in the DL. When the next TIM group begins, the STAs belonging to the previous TIM group that were not served will need to wait for their own TIM group to be addressed again by a TIM beacon and possibly resend the PS-Poll during the following period. In essence, a STA may need to send PS-Poll several times to get the DL data the AP has repeatedly indicated in the TIM beacon of the STA's group. Such unsuccessful and wasted attempts to send the PS-poll or similar naturally consumes power in vain and may also generate interference in the network.